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In our April 16 article “3G and 4G Wireless Speed Showdown,” we reported the results of our exclusive 13-city tests of the four national wireless services: AT&T, Sprint, T-Mobile, and Verizon. Our study is the largest and best-known independent test of wireless service in the United States.

Here, in Part Two of our story, we drill deeper into the massive amounts of data that we collected over six weeks in February and March of this year. Whereas the first article reported the 13-city average speeds of the carriers, in this story we detail the carriers’ performance in each of the ten testing locations we visited in each city.

We’ve assembled a full-page map showing all 127 of our testing locations in Atlanta, Boston, Chicago, Dallas, Denver, Las Vegas, Los Angeles, New Orleans, New York, San Francisco, San Jose, Seattle, and Washington, D.C. When you zoom in on a particular city, you’ll see ten blue map markers–one for each place we tested. Click the pin, and you’ll see our full testing results for that location.

T-Mobile’s HSPA+ 21 service dominated the 3G testing, earning the highest marks for upload speed and download speed in 11 of the 13 cities we studied. The exceptions were Chicago (where AT&T captured the best download and upload speeds for 3G) and Dallas (where AT&T took the lead for 3G downloads, and Verizon won for 3G uploads).

We saw three major patterns in our 4G test results. In five cities (Atlanta, Boston, Las Vegas, Los Angeles, and San Francisco), AT&T swept the 4G upload and download competitions; in four (Denver, New Orleans, San Jose, and Seattle), Verizon swept both; and in the remaining four (Chicago, Dallas, New York, and Washington, D.C.), AT&T won for downloads and Verizon prevailed for uploads. Notably, in two of the cities where Verizon ruled (Denver and Seattle), AT&T doesn’t offer 4G LTE service.

Web Page and Video Load Times

In addition to the speed results, you can see the results of our “behavioral” tests, which are meant to closely approximate the wireless usage of real people. At each of our testing locations, our field researcher loaded a 100KB static Web page and viewed a 1-minute video while connected to each of the 3G and 4G services of the carriers. For each one, the researcher logged the time the Web page took to load, as well as the time necessary to stream the video.

In Video: Demystifying Spotty Cell Phone Coverage

You can see a lot of variance in the performance scores from location to location within our testing cities. The same service may deliver radically different results in two locations in the same part of town. Or two different wireless services might produce very different connection speeds and call quality at the same location.

Thats because a number of variables are constantly affecting the quality of the wireless service you get–factors such as nearby physical structures, your distance from a cell tower, whether you’re connecting from indoors or outdoors, the time of day, and the total number of users connecting to a nearby cell tower at the same time.